Cyanidation of precious metals



Patented Sept. 27, 1938 CYANIDATION OF PRECIOUS METALS Norman Hedley,Old Greenwich, Conn, assignor to American Cyanamid Company, New York, N.Y., a corporation of Maine No Drawing.

6 Claims.

This invention relates to the cyanidation of precious metals.

In the past, the cyanidation of precious metal ores has frequently beencombined with flotation because of the great cheapness of the flotationprocess and the fact that flotation frequently reduces the amount ofcyanide required. With many precious metal ores, therefore, it isstandard procedure to float the ore and treat the flotation concentrateby cyanidation. The most common flotation promotors for precious metalsare the xanthates but unfortunately xanthates exert an inhibiting actionon the solution of precious metals in cyanide so that losses of preciousmetal occur in cyaniding, cyanide consumption is frequently increasedand the time cycle of cyanidation is also increased. It is of coursecommercially impractical to remove the small amounts of xanthate in theconcentrate and therefore an investigation was made to determine whetherthe inhibiting action of the xanthate could be counteracted. During thisinvestigation, numerous compounds were tried but only two proved reallyeffective.

The present invention is therefore based on the use of compoundswhichwill counteract to a greater or less extent the solution inhibitingproperties of xanthates. These compounds are thiourea and thealdehyde-ammonia condensation products thereof.

By means of the present invention, it is possible to increase thesolution rate of precious metals in cyanide from three to five times.The invention is not limited to any particular percentage of addedcompound. As little as 0.1 lb/ton exerts a marked effect and very goodresults are obtained with quantities of about 0.5-2 lbs/ton. Someimprovement is to be noted when the amount used is in excess of 2lbs/ton but the improvement is usually not sufficiently great to warrantthe added reagent cost. It should be understood, therefore, that whilethe best practical range is usually from 0.5 to 2 lbs/ton, the inventionis not limited to this particular range and in every case themetallurgist will adopt the best compromise for any particularconcentrate, balancing reagent cost against added solution.

Both thiourea and an aldehyde-ammonia condensation product of thioureaare very effective. The extent of their action, however, appears to varyslightly. Thiourea has a somewhat lower threshold value, that is to say,its effect in very low concentrations tends to be somewhat greater;

. however, improvement with increased concentration is greater with thealdehyde-ammonia Application February 12, 1938, Serial No. 190,227

condensation product.- Both are, however, very effective and either maybe used alone or mixtures of the two can be employed.

The following examples are given to illustrate the inventionspecifically. The first example is a quantitative .determination of theeffect of the reagents on solution of gold and the second example showsthe results when applied to a typical gold-silver ore. It should beunderstood that the invention is: not limited to the details of theexamples which are given only as representative illustrations of theoperation of the invention.

Example 1 Solution tests were made with a 0.1% sodium cyanide solutionon gold discs having a surface area of 10 cm?, the solution rate beingdetermined by weighing the gold disc after 1 hours and it can beexpressed-as the amount of gold dissolved during that period. A blanktest was made with no xanthate present and then the series of tests with0.051b=/ton of potassium ethyl xanthate and also in the presence of thesame amount of xanthate with various amounts of thiourea oracetaldehyde-ammonia condensation product of thiourea added to thesolution. Temperature, agitation and all other factors were keptentirely constant. The results are shown in the following table:

2 lbs/ton of thiourea increases the solution rate about three times. Itwill be noted that the higher amounts of aldehyde-ammonia thiourea givestill higher increases in solution rate.

, Example 2 The effect of thiourea was tested on a goldsilver ore. Theore was a quartz carrying pyrite and free gold and having the followinganalysis:

Au oz/ton 0.708 Ag ozs/ton 3.35 Fe per cent 1.98 Pb do 0.05 Zn do 0.10Cu do 0.10 S do 2.07 Insol do 88.01

Assay of residue X81} Thiow after cyanida- Extraction thate rea nonadded added Au Ag Au Ag Per Per Lbs/ton Lbs/ton Ozs/ton O2s/ton centcent 0.022 0.33 96. 9 90. 2 0. 110 0. 41 84. 5 87. 7 Pot. ethyl 0.500.50 0.074 0.37 89. 6 89.9 Pot. Sec. buty 0. 0. 214 0.72 69. 8 78. 6Pot. sec. butyl. 0.50 0. 50 0.133 0.42 81.3 87. 5

It will be noted that in each case the extraction of gold and silver ismarkedly increased by the addition of thiourea, the increase beingparticularly striking in the case of the potassium secondary butylxanthate which appears to exert a greater inhibiting action. This is ofspecial importance as this flotation reagent is very largely used inprecious metal ores because of its increased promoting power over theethyl xanthate. I

The mechanism by which thiourea and aldehyde-ammonia condensationproducts act in the present invention is not definitely known. Ibelieve, however, that the effect is primarily one on the surface of themetal. However, the invention is not intended to be limited toany theoryof action.

What I claim is:

1. A method of recovering precious metals by cyanidation of preciousmetal pulps containing xanthates, which comprises carrying out thecyanidation in the presence of a small amount of a compound included inthe group consisting of thiourea and aldehyde-ammonia condensationproducts of thiourea.

2. A method of recovering precious metals by cyanidation of preciousmetal pulps containing xanthates, which comprises carrying out thecyanidation in the presence of a small amount of thiourea.

3. A method of recovering precious metals by cyanidation of preciousmetal pulps containing xanthates, which comprises carrying out thecyanidation in the presence of a small amount of an aldehyde-ammoniacondensation product of thiourea.

4. A method according to claim 1 in which the amount of the compound isbetween 0.5 and 2 lbs/ton, based on the solids of the pulp.

5. A method according to claim 2 in which the amount of thiourea isbetween 0.5 and 2 lbs/ton, based on the solids of the pulp.

6. A method according to claim 3 in which the amount of thealdehyde-ammonia condensation product of thiourea is between 0.5 and 2lbs/ton, based on the solids of the pulp.

NORMAN HEDLEY.

